On Tue, Aug 18, 2015 at 7:46 PM, Andres Freund <and...@anarazel.de> wrote:
> On 2015-08-18 13:24:54 -0400, Peter Eisentraut wrote: > > On 8/18/15 12:35 PM, Robert Haas wrote: > > > If archive_mode=on or max_wal_senders>0, then we need at least > > > wal_level=archive. Otherwise wal_level=minimal is enough. > > > > Totally forgot about max_wal_senders. > > > > However, the thread I linked to earlier aimed for a different master > > plan (or if not, I'm aiming for it now). There is camp 1, which wants > > to keep all the defaults the same, for "performance" or something like > > that. And there is camp 2, which wants to have a replication-friendly > > setup by default. Instead of fighting over this, your idea was to be > > able to switch between 1 and 2 easily (which means in particular without > > restarts). > > I don't think not requiring restarts is sufficient, having to twiddle a > bunch of parameters manually still is a lot more effort than people see > as necessary. > > The only reason I think changing the default is a good approach is that > it's doable within a relatively short amount of time. > > > But if we tie the effective wal_level to archive_mode or > > max_wal_senders, both of which are restart-only, then we haven't gained > > anything. (We would have removed half a GUC parameter, effectively. > > Not bad, but not very exciting.) > > ISTM that it's not too hard to > a) make archive_mode PGC_SIGHUP > b) make wal_level PGC_SIGHUP > c) automatically increase wal_level to logical whenever a logical > replication slot is defined > > it seems considerably harder to > > d) make max_wal_senders PGC_SIGHUP > e) make max_replication_slots PGC_SIGHUP > > because they need shmem, locks, and everything. > > > Therefore I propose something slightly different: > > We change the default of max_wal_senders, max_replication_slots, to some > reasonably high setting and make wal_level an internal automagically > determined variable. archive_mode = on automatically increases the level > to what's now hot-standby. > > To deal with streaming replication, we automatically create slots for > replicas, but, by default, *without* having them reserve WAL. The slot > name would be determined in some automatic fashion (uuid or something) > and stored in a new file in the data directory. That allows us to > increase the internal wal_level to hot_standby/archive whenever a > replica has connected (and thus a physical slot exists), and to logical > whenever a logical slot exists. > What happens "the first time"? Meaning I'm on wal_level=minimal and take a base backup. Then when the replica first connects 10 minutes later, it needs WAL back in time, which was logged at wal_level=minimal. So you'd need to bump it up whenever a base backup is done -- but then you can't drop it back down again or your base backup will be useless. Or am I missing something? -- Magnus Hagander Me: http://www.hagander.net/ Work: http://www.redpill-linpro.com/
